Dedication page i Dedication page This thesis is dedicated to the memory of my grandmother,

Abstract ii A polymorphism suggestive of a division of labour has recently been observed in colonies of some species of parasitic trematodes. In these colonies there are two distinct castes: one which reproduces and one which is unable to reproduce and provides benefits to the colony in some other way. This recently uncovered trait has been reported from a number of closely related trematode species. It has been hypothesised, based on caste member behaviour as well as parallels with the division of labour in social insects, that the division of labour in trematodes is an evolutionary response to competitive interactions, and that the role of the non-reproductive caste is to defend the colony against co-infecting trematodes. The aim of this thesis was to further understand this division of labour in Philophthalmus sp. colonies within the first intermediate snail host, Zeacumantus subcarinatus. I aimed to determine fitness benefits gained by colonies that invest in non-reproductive caste members as well as conditions under which colony organisation and division of labour changes. This was done using a range of manipulative and observationa